I use a similar approach at work (though with a vastly smaller codebase ~ only 20k lines or so). I don't get why they're using maven though; sbt is amazing; especially with 0.13.2 and the new hash algorithm, not to mention all of the great plugins it brings.

I've only used Scala briefly (and then sbt fulfilled my meager needs) but from the rants by people I follow, each one of them shifted between sbt and mvn every 1-n releases over the years, because the one they were currently using annoyed them too much.

Then again I guess this goes hand in hand with the old criticism that the various releases are not backwards compatible.

We rely on several maven plugins without equivalents in sbt.